﻿Properties 
  of 
  Films 
  of 
  Magnetic 
  Metals. 
  

  

  837 
  

  

  was 
  found 
  that 
  a 
  wave-length 
  of 
  about 
  430 
  ja/j, 
  fulfilled 
  this 
  

   condition 
  very 
  satisfactorily, 
  inasmuch 
  as 
  this 
  colour 
  gave 
  

   generally 
  the 
  most 
  distinct 
  bands 
  of 
  alL 
  

  

  3. 
  Optical 
  Thickness 
  of 
  Films* 
  

  

  The 
  refractive 
  index 
  of 
  the 
  films 
  was 
  obtained 
  as 
  the 
  ratio 
  

   between 
  the 
  optical 
  thickness 
  and 
  the 
  air 
  thickness 
  — 
  that 
  is, 
  the 
  

   ratio 
  of 
  the 
  number 
  of 
  wave-lengths 
  in 
  the 
  film 
  to 
  that 
  in 
  the 
  

   same 
  thickness 
  of 
  air. 
  The 
  optical 
  thickness 
  was 
  measured 
  by 
  

   means 
  of 
  an 
  interferometer 
  of 
  the 
  Zehnder 
  type, 
  following 
  

   the 
  method 
  used 
  at 
  different 
  times 
  in 
  this 
  laboratory 
  *. 
  Fig. 
  2 
  

  

  presents 
  a 
  diagram 
  of 
  the 
  system. 
  Light 
  from 
  a 
  Nernst 
  

   glower 
  S 
  (or 
  slit 
  illuminated 
  with 
  sunlight) 
  passes 
  through 
  

   a 
  lens 
  Li, 
  a 
  polarizing 
  nicol 
  N, 
  and 
  is 
  divided 
  into 
  two 
  com- 
  

   ponents 
  at 
  the 
  half-silvered 
  surface 
  of 
  the 
  interferometer- 
  

   plate 
  A. 
  These 
  rays 
  pass 
  respectively 
  over 
  the 
  two 
  paths 
  

   ABC 
  and 
  AB'C, 
  and 
  recombine 
  beyond 
  the 
  interferometer- 
  

   plate 
  C 
  under 
  conditions 
  producing 
  interference. 
  The 
  object 
  

   of 
  the 
  nicol 
  is 
  to 
  eliminate 
  reflexion 
  from 
  the 
  glass-air 
  surface 
  

   of 
  the 
  interferometer-plates, 
  and 
  the 
  consequent 
  blurring 
  of 
  

   the 
  bands 
  arising 
  from 
  the 
  superposition 
  of 
  the 
  two 
  sets 
  thus 
  

   produced. 
  This 
  is 
  possible 
  because 
  the 
  angle 
  of 
  incidence 
  

   on 
  the 
  plates 
  is 
  very 
  nearly 
  the 
  polarizing 
  angle. 
  The 
  film 
  F 
  

   whose 
  retardation 
  is 
  to 
  be 
  measured 
  is 
  placed 
  to 
  intercept 
  the 
  

   lower 
  half 
  of 
  one 
  of 
  the 
  component 
  rays. 
  Just 
  beyond 
  this 
  

   is 
  placed 
  a 
  compensator 
  K, 
  consisting 
  of 
  two 
  thin 
  mica 
  strips 
  

   of 
  equal 
  thickness, 
  one 
  placed 
  parallel 
  to 
  the 
  film 
  with 
  its 
  

   upper 
  edge 
  in 
  line 
  with 
  the 
  upper 
  edge 
  of 
  the 
  film, 
  the 
  other 
  

   placed 
  to 
  intercept 
  the 
  part 
  of 
  the 
  ray 
  which 
  passes 
  abovcj 
  the 
  

   film 
  and 
  the 
  companion 
  strip 
  of 
  mica. 
  Both 
  have 
  their 
  optic 
  

  

  * 
  Cartmel, 
  Phil. 
  Mag. 
  [6] 
  vi. 
  p. 
  214 
  ; 
  Clark, 
  Phys. 
  Ptev. 
  xxiii. 
  p. 
  422. 
  

  

  